1. Field of the Invention
The present invention pertains to a dry type vacuum pump comprising a device for sealing against lubricants mounted between a lubricated support bearing and a pumping stage. The invention can be applied especially to a Roots or claw dry type vacuum pump comprising two rotary lobed shafts or again a spiral type or screw type pump or a pump working on another similar principle.
These pumps generally comprise one or more series-mounted pumping stages in which a gas to be pumped flows between an admission intake and a delivery outlet. Prior-art vacuum pumps include rotary lobe pumps known as bi-lobe or tri-lobe Roots pumps and double claw pumps also known as “claw” pumps.
A rotary lobe pump comprises two rotors of identical profiles rotating in opposite directions inside a stator. During rotation, the gas to be pumped is trapped in the free space contained between the rotors and the stator, and is driven by the rotor to the following stage or after the first stage to the delivery outlet. The operation takes place without any mechanical contact between the rotors and the stator, thus making it possible to have total absence of oil in the pumping stages.
The rotors are borne by the rotary shafts supported by lubricated support bearings of a shaft-end-mounted motor drive compartment. This motor drive compartment is isolated from the pumping stages by a device for sealing against lubricants through which the rotary shafts are always liable to rotate.
During operation, the rotating of the shafts in the lubricated support bearings gives rise to a lubricant mist which, when it undergoes pressure variations, risks migrating toward the pumping stages. Now, it is indispensible that no trace of oil or grease should be found at the pumping stages for applications known as “dry” operations such as semiconductor substrate manufacturing methods.
2. Description of the Prior Art
There already exist devices in the prior art for sealing against lubricants comprising a lubricant deflector or baffle and a frictional ring seal known as a lip seal. The lubricant deflector is mounted on the rotary shaft between the lubricated support bearings and the frictional ring seal and rotates connectedly with the rotary shaft during operation. The deflector deflects the lubricant by the effect of centrifugal force and sends it to the bottom of the motor drive compartment through a small pipe, the inlet of which is positioned facing the end of the lubricant deflector and the outlet of which opens into the bottom of the motor drive compartment. This device keeps the lubricant confined within the motor drive compartment. The frictional ring seal forms a second security element if lubricant residues should nonetheless pass through the lubricant deflector. However, this safety measure could prove to be inadequate. The intensification of production output rates is leading to increased operating temperatures for vacuum pumps, and this can make the frictional ring seal more brittle. Furthermore, the increase in the repetition of pressure variations on either side of the frictional ring seal, which can be associated with corrosiveness in the lubricant that has gone through the lubricant deflector, can cause a premature wearing out of the frictional ring seal and require servicing at closer intervals, each intervention involving the stoppage of the semiconductor manufacturing installations and stoppage of the vacuum pump, and this is very costly.